And some forecast models show the heat index could be greater than 105 degrees this weekend -- high enough to increase health risks.

"The heat index tells you how it feels outside in the shade," said the Maryland Department of Health and Mental Hygiene in a statement. "The heat index is a measurement of how it feels when relative humidity is combined with the effects of the air temperature."

It's been a while since the temps were so high: July 26, 2012 was the last recorded 100-degree day in the D.C. metro region.

That year, the Center for Disease Control and Prevention reported a total of 32 heat-related deaths, including 12 in Maryland and 12 in Virginia from June 30 to July 13.

But it hasn't exactly been cool this summer, either. Tomorrow marks the 18th 90-plus degree day this year. We hit the 90s for the first time this year on June 11.

According to the Virginia Department of Health, the best ways to avoid heat-related illness is to:

• Drink plenty of water and avoid drinks that have a lot of caffeine or large amounts of sugar.